Direct bonding of metals with a metal-gas eutectic

ABSTRACT

A method is described for direct bonding of metallic members to other metallic members with a metal-gas eutectic. The method comprises placing a metal member such as copper, for example, in contact with another metal member, such as nickel, for example, heating the metal members to a temperature slightly below the melting point of the lower melting point metal, e.g., approximately 1072*C. for copper, the heating being performed in a reactive atmosphere, such as an oxidizing atmosphere, for a sufficient time to create a metal-gas eutectic melt which, upon cooling, bonds the metal members together. Various metals and reactive gases are described for direct bonding.

United States Patent [19 Burgess et a1.

m I 3,854,892 1 Dec. 17, 1974 [22] Filed:

[ DIRECT BONDING OF METALS WITH A METAL-GAS EUTECTIC [73] Assignee:General Electric Company,

Schenectady, NY.

Mar. 1, 1973 21 Appl. No.: 337,143

Related U.S.- Application Data [62] Division of Ser. No. 245,890, April2(1, 1972, Pat.

[52] U.S. Cl 29/l96.l, 29/194, 29/1962, 29/l96.3, 29/l96.6,' 29/196,29/197,

[51] Int. Cl B32b 15/00, B32b 15/20 [58] Field of Search 29/494, 196.1,196.2, 196.3,

[56] References Cited Simpelaar 29/194 X 3,093,885 6/1963 Morrison etal. 29/l96.2 3,322,517 5/1967 Miller 29/l97.5 3,667,110 6/1972 PrimaryExaminerC. Lovell Assistant ExaminerArthur J. Steiner Attorney, Agent,or FirmPaul F. Wille; Joseph T. Cohen; Jerome C. Squillaro [571 ABSTRACTA method is described for direct bonding of metallic.

members to other metallic members with a metal-gas eutectic. The methodcomprises placing a metal member such as copper, for example, in contactwith another metal member, such as nickel, for example, heating themetal members to a temperature slightly below the melting point of thelower melting point metal, e.g., approximately 1072C. for copper, theheating being performed in a reactive atmosphere, such as an oxidizingatmosphere, for a sufficient time to create a metal-gas eutectic meltwhich, upon cooling, bonds the metal members together.- Various metalsand reactivegases are described for direct bond- 7 Claims, 3 DrawingFigures Gwyn 29 494 PATENTS} B531 3,854,892

PLACE METAL MEMBERS //V CONTACT WITH EACH OTHER HEAT //v REACT/V5ATMOSPHERE 7'0 1 FORM EUTECT/C EUTECT/C MELT WETS METAL MEMBERS COOL TOFORM BOND BE TWE E /V METAL ME MBE RS GAS OUTLET DIRECT BONDING OFMETALS WITH A METAL-GAS EUTECTIC This is a division, of application Ser.No. 245,890, filed Apr. 20, 1972 now U.S. Pat. No. 3,744,120.

The present invention relates to improved bonds and methods of directlybonding two or more metallic members together. This application relatesto our concurrently filed application Ser. No. 245,889, now US. Pat. No.3,766,634, of common .assignee. The entire disclosure of which isincorporated herein by reference thereto.

The formation of bonds between metallic members is achieved in various,ways. For example, certain metals can be bonded together with the use ofsolders. Other metals are bonded together by welds, such as are welds orspot welds. Where certain metals cannot be directly bonded to eachother, generally intermediate metallic members are used to form thebond. The need for simple methods of forming bonds between similar anddissimilar metals still exists. For example, in the fabrication ofsemiconductor integrated circuits, tenacious bonds between variousmetals arerequired. In addition, it is desirable to provide low ohmiccontact between such metals. The foregoing methods are frequently notcompatible with integrated circuit fabrication and even if compatiblearefrequently economically unacceptable. Accordingly, a need for asimple and'economically acceptable method of forming bonds betweenmetallic members is still desired.

It is therefore an object of this invention to provide a method offorming bonds between metallic members with a metal-gas eutecticcomposition.

It is yet another object of this invention to provide a method ofbonding metallic members together without the use of intermediate metallayers.

Another object of this invention is to provide a method of bondingmetallic members together in a simple heating step without the need forintermediate flux.

Yet another object of this invention is to provide a tenacious bond anda method of forming this bond between metallic members which bondexhibits low ohmic resistance and is compatiblewith the fabrication ofsemiconductor integrated circuit modules.

Briefly, our invention relates to bonds and methods of bonding togethermetallic members by placing at least two metallic members in contactwith each other and elevating the temperature of the members in areactive atmosphere of selected gases and at controlled partialpressures for a sufficient time to produce a metal-gas eutecticcomposition on the surface of at least one of the metallic members. Thiseutectic composition or melt forms at a temperature below the meltingpoint of one of the metallic members and wets both metallic members sothat upon cooling, a tenacious bond is formed between themetallic-members. By way of ex-' become more apparent to those skilledin the art from the following detailed description taken in connectionwith the accompanying drawings in which:

FIG. 1 illustrates a typical bond between two metallic members formed inaccord with our invention;

FIG. 2 is a flow diagram illustrating the process steps for formingtenacious metal-to-metal bonds in accord with our invention; and i aFIG. 3 schematically illustrates a horizontal furnace usefulinpracticing our invention.

FIG. 1 illustrates, by way of example, a typical bond 11 betweenmetallic members 12 and 13. The bond 11 comprises a eutectic compositionformed with at least one of the metallic members and a reactive gas inaccord with the novel aspects of our invention.

As used herein, the term metallic member or material is intended toinclude such materialsas copper, nickel, iron, cobalt, chromium, silver,aluminum, alloys of the aforementioned elemental materials, andstainless steel. As will become more apparent'from the followingdescription, still other metallic materials, such as beryllium-copper,for example, may also be, advantageously employed, if desired.

,The term eutectic'or metal-gas eutectic composition as used hereinmeans a mixtureof atoms of the metallic member and the reactive gasor'compound formed between the metal and the reactive gas; but does notinclude eutectics formed by the reaction or mixing of two metals ratherthan a reaction between a metal and a ,component of a gas. For example,where the-metallic member is copper and the reactive gas is oxygen, the

member is cobalt and the reactive gas is a sulfur-' bearing gas, theeutectic is formed between cobalt and cobalt sulfide.

The novel process for making tenacious bonds between metallic members 12and I3 is illustrated inthe flowchart of FIG. 2. More specifically, FIG.2'illustrates the practice of our invention by placing two metallicmembers in contact with each other, such as one member overlyinganother. These members are then placed in a suitable furnace, such as isdescribed below, which includes a reactive atmosphere 'such that uponheating of the metallic members, a.metal-gas eutectic composition forms.The temperature-at which the desired eutectic composition forms and thepartial pressure of the'reactive gas necessary to form the desiredeutectic composition depend upon the selected metallic members and thereactive gas. lngeneral, however; the partial pressure of the reactivegas must exceed the equilibriumpartial pressure of the reactive gas inthe metal at or above the eutectic temperature. For example, whenbonding copper members together, a reactive atmosphere including oxygen,for example, re-

quires a partial pressure of oxygen in excessof 1.5 X

' 10' atmospheres at the eutectic "temperature of l065C. Other metallicmaterialsand other reactive gases require different partial pressuresand different temperatures to form the desired eutectic.

Table I is a representative listing of typical eutectic compositionswhich are useful in practicing our invention. These eutectics are formedby reacting themetallic members to be bonded with a reactive gascontrollably introduced into an oven or furnace.

. TABLE 1 Per Cent by Weight Metal-Gas Eutectic of Reactive Gas EutecticTemperature, C. at Eutectic Composition iron-Oxygen v l523 O. l 6 OCopper-Oxy en lO65 0.39 Chromiumygen l800 0.6 0 Chromium-Sulfur 1550 2.2S Copper-Phosphorus 714 8.4 P Nickel-Oxygen 1438 0.24 I ONickel-Phosphorus 880 11.0 P Molybdenum-Silicon 2070 5.5 SiSilver-Sulfur 906 1 .8 V S Silver-Phosphorus 878 1 .0 P Copper-Sulfur1067 0.77 S Co alt-Oxy en l45l 0.23 0

Aluminumilicon 577 11.7 Si

The eutectics listed in Table I are formed by reacting the metallicmembersin an oxygen-bearing gas,-such as oxygen, a sulfur-bearing gas,such as hydrogen sulfide, a phosphorus-bearing gas, such as phosphine,or a silicon-bearing gas, such as silane. At the eutectic temperature ofthe selected. metallic member and the reactive gas, such as thosetemperatures listed inTable l, the eutectic composition becomes a'liquid and wets the adjoining member so that upon cooling, the metallicmembers become tenaciously bonded together.

Table, ll illustrates, by way of example, typical metalto-metal bondsformed in accord with our invention and the conditions under which thebonds are formed. For these conditions, the reactive gas is oxygen.

TABLE'II Time at Temperature, Elevated Metals 1 Thickness C TemperatureCu Cu 5 mils lO72C 0.5 hrs. Cu Ni 5 mils 1072C LO hrs. Cu-StainlessSteel 5 mils 1072C 1.0 hrs. I i i IO mils 1445C 1.0 hrs.

' Fe Fe It) mils 1530C 1.0 hrs.

- Co C0 l5 mils 1458C in'Table ll are byway of example',and not by wayof tallic member, for example, themetallic member tends to plasticallyconformto the shape of theother member and thereby produce better bondsthan those eutectics which become liquids at temperatures greaterthanapproximately 50C. belowthe meltingpoint of the metallic member/Theuniformity -of the bond therefore appears to be related to the creep ofthe metal which becomes considerableonly near the melting point.

' From Table -I, for example,-it canbe seenthat-the folour invention andthe methods of forming metal-tometal bonds, apparatus usefulin'practicing our invention along with more specific details oftheproceSs will now be describedwith'reference to FIG. 3.-

FIG. 3 illustrates a horizontal furnacecomprising an elongated quartztube 22, for example, having 'a gas other end. The quartz tube'22 alsoincludes an opening Y The examplesof metal-to-metal bonding illustratedlimitation. In general, most metals which form a metah gas eutectic .ina reactive atmosphere are useful in forming metal-to-metal bonds.- Thebonding can be between like metals, dissimilar metals, or even alloys.For

example, where like metals are bonded together, the

eutectic forms on both surfaces of the members. Where dissimilar metalsare bonded together, the eutectic generally forms on at least onesurface of the metal having the lower eutectic-forming temperature. Forexample, as in the case of copper-nickel, the eutectic forms with thecopper. The eutectic then wa both metal surfaces thereby forming thedesired bond. Where alloys are" employed, such as the various alloys ofnickel, iron, cobalt, copper, silver, chromium'and aluminum are employed,-the eutectic composition is believed to form' with one of. theelemental metals, generally the one withthe lower melting point;

inlet 23 atone end thereof and a gas outlet24 at the or port 25 throughwhich materials are placed into and removed from the furnace. Materialsare placed on a' holder 26 having a push *rod- 27-extending'throughoneendof the.furnace so that the holder and materials placed thereon may beintroduced and removed from thefurnace. r

. The furnace 21 .is also providedwith suitable heating .elements,illustrated-inFlG.-3'.as electrical wires 28 which surround the quartz:tube 22 in the region to be' heated. The electrical wires 28 may, forexample, be

. connected to a suitable current source, such as a 220- voltalternating currentsource. The electrical wires 28 may then besurrounded by suitable insulatin'grnaterial Y 29 to confine the heatgenerated by the electrical'wires to the region within, the quartz tube.Obviously those skilled in the art can appreciate that" other. heati-ng'means may also be employed, if desired, and that FIG.

3 is merely illustrative of one such heating means T'he temperature ofthe furnace is detected by' a suitable thermocouple 29 whichextendsthrough an opening in the quartz tube soth'at electricalconnections canbe made thereto. FIG. 3 also illustrates a metallic .member 12positioned on theholder 26 and a metallic member One factor whichappears to affect the tenacity and uniformity of metal-to-metal bondsformed in accord with our invention-is the relationshipbetween themelting point of the metallic member andtheeutectic tem-' perature.Where the eutectic temperature is within apflow of approximately 4 cubicfeet per hour of nitrogen l3 overlying the member 12. These metallicmembers are introduced into the quartz tube through theopening 25 whichis then sealed by suitablestopper means.

The quartz tube 22 is then purged with a reactive gas and-0.02 cubicfeet per hour of oxygen, for example; As

-' used herein, reactive gas flow or. atmosphere means a mixture of aninert gas such as argon, helium, or nitrothe thickness of the materials,and the gas flow rate, in

a manner more fully described below. In general, however, the partialpressure of the reactive gas must exceed the equilibrium partialpressure of the reactive gas in the metal at or above the eutectictemperature. As

pointed out above, when bonding copper members together, a reactiveatmosphere including oxygen, for example, the partial pressure of oxygenmust be in excess of 1.5 X atmospheres at the eutectic temperature of1065 C.

After purging the quartz tube, the furnace is then i brought to atemperature sufficient to form a eutectic melt at the metal-to-metalinterface. For example, for

a copper-nickel bond with oxygen as the reactive gas,

the temperature of the furnace is brought to approximately lO72C. Atthis temperature, a copper-copper oxide eutectic forms on the coppermember and wets the copper member and the nickel member so that uponcooling, a tenacious bond is formed between the two metals.

In general, the times necessary to form this eutectic melt range betweenapproximately 10 minutes for 1- mil-thick copper members andapproximately 60 minutes for 250-mil-thick copper members, for example.For metallic members of other thicknesses and geometric configurations,the times required to form the eutectic melt vary. In general, thelonger the metallic members are held at the eutectic temperature, thethicker the eutectic will be. The thickness of the eutectic also dependsupon the partial pressure of the reacting gas. As pointed outpreviously, a partial pressure below the equilibrium partial pressurefor the specific eutectic will result in no eutectic formation. Hencepartial pressures in excess of this equilibrium value are required toproduce the desired eutectic. If the partial pressure of the reactinggas is too high, however, all the metal reacts with the reactive gas andforms, for example, an oxide, sulfide, phosphide, etc. which preventsthe formation of the eutectic melt. Thus, an intermediate reacting gaspartial pressure is required so that both the eutectic melt phase andthe metallic phase-are present simultaneously. Tests have illustratedthat extremely strong bonds are achieved when both phases are present.Accordingly, in practicing our invention the partial pressure of thereacting gas must be sufficiently great to permit the formation of aeutectic with the metal but not so great as to completely convert themetal to the oxide, sulfide, phosphide, etc. during the bonding time.

We have found that consistently good bonds are achieved between metallicmembers so long as the aforementioned conditions are met; However, nobonding occurs where the partial pressure of the reactive gas is lessthan the equilibrium partial pressure at the eutectic temperature and nobonding occurs where the partial pressure of the reactive gas is suchthat all the metallic member is converted to an oxide, phosphide,sulfide, etc.

Also, those skilled in the art can appreciate that the gas flow rate isnot critical to the practice of our invenof example, selected metals andthe percent of reactive tion and may'be-varied over wide ranges withoutmate- I rially affecting the integrity of the bonds. However,

economic considerations will generally control the acceptable gas flowrates. Further, the partial pressure of the reactive gas-in the inertgas also can be varied, de-

pending in part on the relative sizes of the materials to be bonded. Thegas flow rate and the presence of reactive elements in the flow system,such as carbon susceptors, the presence of residual oxygen or water inthe bonding system and the bonding time.

. Table III illustrates useful ranges for partial pressures of reactivegases at whichbonding occurs between selected metals in the presence ofoxygen-bearing or sulfur-bearing gases. Only those eutectics whichexhibit exhibit a eutectic temperature within of the melting point ofthe metal are listed. I

TABLE III EU'racnc REACTIVE GAS COMPOUND -BY VOLUME a cu-cuo 0.01 0;5 CuCuS 0.01- 0.5 Ni NiO 0.01 0. 3 C0 C00 0.01 0.4 'FeFeO 0.01-0.3

It is to be understood that Table III illustrates, by way gases in thetotal gas flow which are-useful in practicing our invention. However,those skilled in the art can readily appreciate that other materials andother reactive atmosphere of oxygen. Additionally, useful bonds areformed with molybdenum or aluminum in a reactive atmosphere includingsilane. Accordingly, it is to be understood that Table III is merely apartial listing of eutectic compounds and that our invention is notlimited solely to those eutectics set forth in Table III.

Those skilled in the art can readily appreciate that the formation ofmetal-to-metal bonds with a metal-gas eutectic provides an extremelyuseful capibility in electrical and electronic systems. For example,metal-tometal bonds may be used for interconnections, packaging ofelectronic components, formation of hermetic seals, electrical crossovesin integrated circuits, to mention only a few. These metal-to-metalbonds are formed without the use of compressive forces on the metalmembers and do not require the interdiffusion of metals to effecttenacious bonds. Additionally, although the metallic members areillustrated as sheets or plates,

it is to be understood that other configurations may also be used in thepractice of our invention. Still other changes and modifications willoccur to those skilled in the art and hence, the appended claims areintended to cover all such changes and modifications as fall within asecond metal member; I

a eutectic bond between said first and second metal 4. The structure ofclaim 1 wherein at least said first metal members is copper and saideutectic bond is copper-copper oxide.

5. The structure of claim 4 wherein said second metal 5 member isselected from the group consisting of copper, nickel, cobalt, chromium,iron and alloys thereof. 6. The structure of claim 4 wherein said secondmetal member is stainless steel.

7. The structure of claim 1 wherein said first metal member is aluminumand said eutectic is aluminum-" aluminum silicide.

1. A BONDED METAL-TO-METAL STRUCTURE COMPRISING: A FIRST METAL MEMBER; ASECOND METAL MEMBER; A EUTECTIC BOND BETWEEN SAID FIRST AND SECOND METALMEMBERS SAID EUTECTIC BOND CONSISTING ESSENTIALLY OF A MIXTURE OF ATOMSOF AT LEAST SAID FIRST METAL MEMBER AND ONE OF THE GROUP CONSISTING OFOXIDES, SULFIDES, PHOSPHIDES AND SILICIDES OF SAID METAL.
 2. Thestructure of claim 1 wherein said metal members are selected from thegroup consisting of copper, nickel, cobalt, chromium, iron, silver,aluminum, alloys thereof and stainless steel.
 3. The structure of claim2 wherein at least one of said metal members is copper.
 4. The structureof claim 1 wherein at least said first metal members is copper and saideutectic bond is copper-copper oxide.
 5. The structure of claim 4wherein said second metal member is selected from the group consistingof copper, nickel, cobalt, chromium, iron and alloys thereof.
 6. Thestructure of claim 4 wherein said second metal member is stainlesssteel.
 7. The structure of claim 1 wherein said first metal member isaluminum and said eutectic is aluminum-aluminum silicide.